Process for the production of a ceramic polycrystalline abrasive

Abstract

A process for the production of a ceramic polycrystalline abrasive by the sol/gel method is characterized in that an intermediate containing aluminium hydroxide is mixed, dried, calcined and sintered with a further intermediate having different degradation properties. The further intermediate is expediently formed from a calcined aluminium hydroxide.

We claim: 
     
       1. A process for the production of a ceramic polycrystalline abrasive by the steps of dispersing a hydrated alumina in water followed by dewatering, calcining and sintering, wherein the improvement comprises: said step of dispersing includes forming a dispersion which simultaneously contains particles of aluminum hydroxide, particles of gamma phase alumina, and dissolved magnesium salt, and   said step of forming a dispersion is followed by said steps of dewatering, calcining and sintering.   
     
     
       2. The process of claim 1, wherein the steps of forming a dispersion, dewatering, and calcining include said magnesium forming a spinel associated with gamma phase alumina. 
     
     
       3. The process of claim 1, wherein after the step of calcining, said magnesium has formed a spinel associated with gamma phase alumina. 
     
     
       4. The process of claim 1, wherein the step of forming a dispersion includes, forming a first dispersion in water of a first material containing particles of aluminum hydroxide,   forming a second dispersion in water of a second material containing particles of gamma phase alumina, and   combining the first and second dispersions.   
     
     
       5. The process of claim 4, wherein the step of forming a dispersion includes mixing dissolved magnesium salt in at least one of the first and second dispersions before the step of combining. 
     
     
       6. The process of claim 4 wherein the step of forming a dispersion includes adding a solution of magnesium nitrate and water to the combination of the first and second dispersions. 
     
     
       7. The process of claim 1, wherein the step of forming the dispersion includes mixing the aluminum hydroxide particles with the particles of gamma phase alumina and adding a magnesium salt during said mixing. 
     
     
       8. A process according to claim 1, characterized in that the particles of gamma phase alumina are contained in a calcined aluminum hydroxide not completely converted into alpha-alumina. 
     
     
       9. A process according to claim 1, characterized in that the particles of gamma phase alumina are ground before being dispersed with the particles of aluminum hydroxide and the dissolved magnesium salt. 
     
     
       10. A process according to claim 1, characterized in that the particles of aluminum hydroxide are a finely dispersed aluminum metahydroxide. 
     
     
       11. A process according to claim 1, characterized in that the particles of aluminum hyroxide are dispersed with the particles of gamma phase alumina in such a proportion that their contents of anhydrous aluminum oxide are, respectively, in the ratio from 99:1 to 80:20. 
     
     
       12. A process according to claim 1, characterized in that the proportion of aluminum in said particles of aluminum hydroxide and gamma phase alumina together, is at least 90%. 
     
     
       13. A process according to claim 12, characterized in that said proportion of aluminum is at least 99%. 
     
     
       14. A process according to claim 1, characterized in that the magnesium content in the dispersion of the particles of aluminum hydroxide and particles of gamma phase alumina corresponds to a molar ratio of MgO to Al 2  O 3  of at least 0.1 in the sintered product. 
     
     
       15. A process according to claim 1, characterized in that the step of forming a dispersion is followed by the step of forming a gel, and the gel is extruded prior to dewatering. 
     
     
       16. A process for the production of ceramic polycrystalline corundum abrasive, comprising: forming a dispersion in water, the dispersion containing particles of aluminum hydroxide, particles of gamma phase alumina, and magnesium ions, whereby said ions and particles of alumina form a nucleating material;   dewatering the dispersion to form a calcined matrix in which spinel is distributed at the nucleating material; and   heating the calcined matrix to transform the gamma phase alumina into alpha phase alumina.   
     
     
       17. The process of claim 16, wherein the step of heating the calcined matrix includes heating the matrix to a temperature in the range of 1250°-1450° C. 
     
     
       18. The process of claim 16, wherein the duration of said heating is between 5 and 90 minutes. 
     
     
       19. The process of claim 16, wherein the magnesium salt is magnesium nitrate. 
     
     
       20. A process according to claim 10, characterized in that the aluminum metahydroxide belongs to the boehmite type.